Opto-electronic devices that make use of organic luminescent materials are becoming increasingly desirable for a number of reasons. The organic semiconductor materials found application in inexpensive fabrication processes for organic opto-electronic devices. Such materials and devices have the potential for cost advantages over semiconductor-based inorganic devices. In addition, the inherent properties of organic and organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Example of organic optoelectronic devices include organic light emitting devices (OLEDs), where the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily timed with appropriate dopants.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a luminescent or phosphorescent small molecule emitter. These variations are alos included in the present invention.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
OLED devices are generally (but not always) intended to emit light through at least one of the electrodes. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, may also be used. In a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may be comprised of a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. Where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices where both electrodes are transparent may also be fabricated. Side emitting OLEDs comprising one or both electrodes may be opaque or reflective in such devices may also be fabricated.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between. Accordingly, it will be understood that the invention encompasses any OLED that comprises layers and constituents which are arranged so as to provide a functional OLED, regardless the specific arrangement or orientation of the layers or the device comprising them.
As used herein, “dopant” means a luminescent materials or compound capable of emitting light of a desired wavelength, but is generally (but not always) capable or intended to transport electrical charge. Such dopant is usually dispersed in a host material, which is capable of transporting the charges under a bias voltage.
As used herein, “host” means material, which is generally (but not always) essentially colorless transparent non-emissive organic semiconductor material capable of transporting electrical charge under bias voltage.
The host capable of transporting negative charge is a semiconductor capable of accepting electrons from adjacent layer thus becoming negatively charged. Said negative charge migrates toward anode through the host material under bias voltage. The host capable of transporting positive charge is a semiconductor capable of accepting holes (an equivalent of cation) from adjacent layer thus becoming positively charged.
A bipolar or ambipolar host is a semiconductor capable of accepting both electrons and holes from adjacent layer thus becoming locally negatively and positively charged. The overall charge in the bipolar hosts may be positive if the prevalent charge carrier is a hole, or negative if the prevalent charge carrier is an electron.
As used herein, “doped emissive layer” means material, in which the emissive dopant is dispersed in a host to form a semiconducting blend capable of both transporting the electrical charge under bias as well as emitting light of a desired wavelength that corresponds to the dopant emission spectrum.
One application for luminescent emissive layers is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates or CR1, which are well known to the art.
Industry standards call for the lifetime of such full color displays to be at least about 5000 hours. In addition, high stability and efficiency are important characteristics of high quality displays. These requirements generate a need for organic semiconductor materials useful for fabrication OLEDs exhibiting longer lifetimes, higher stability, and higher efficiency in the red, green and blue wavelength regimes than have been achieved in the prior art.
One example of a host capable of transporting electrons is 1,3,5-tris(phenyl-2-benzimidazolyl)benzene, denoted TPBI, which has the structure of Formula I:

TPBI displays an fluorescence and phosphorescence spectra shown in FIG. 3 corresponding to a triplet energy of 3E=2.67 eV respectively. Shi, J.; Tang, C. W.; Chen, C. H. U.S. Pat. No. 5,646,948, 1997. Gao, Zhiqiang; Lee, C. S.; Bello, I.; Lee, S. T.; Chen, Ruey-Ming; Luh, Tien-Yau; Shi, J.; Tang, C. W. Appl. Phys. Lett. 1999, 74, 865. The unique nature of TPBI provides also for high electron mobility and thermal stability. Wong, T. C.; Kovac, J.; Lee, C. S.; Hung, L. S.; Lee, S. T. Chem. Phys. Lett. 2001, 334, 61.
Second example of a host capable of transporting electrons is N,N′-dicarbazolyl-4,4′-biphenyl, denoted CBP, which has the structure of Formula II:

CBP displays an fluorescence and phosphorescence spectra corresponding to the triplet energy of 3E=2.56 eV respectively. This compound is known in the literature since 1950's. Gilman, H.; Honeycutt, J. B., Jr. J. Org. Chem. 1957, 22, 226. Patented for use in organic electroluminescence Kanai, H.; Sato, Y.; Ichinosawa, A. Jpn. Kokai Tokkyo Koho 1996, JP 08060144. It's use is also described in Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082. The unique nature of CBP provides also for high hole mobility and thermal stability. Parshin, M. A.; Ollevier, J.; Van der Auweraer, M. Proc. SPIE 2006, 6192, 61922A/1-61922A/8.
Since triplets and singlets are generated by electron-hole recombination in a 3:1 ratio, utilizing triplet excited states generated in the host materials by phosphorescent dopants is of utmost importance for obtaining the maximum light output in OLEDs. Baldo, M. A. and Forrest, S. R. Phys. Rev. B. 2000, 62, 10958. Adachi, C. et al. Appl. Phys. Lett. 2001, 79, 2082.
In general, the desired energy of singlet and triplet energy transfer in the emissive layer is from the non-emissive host to the emissive dopant. Therefore the excited state energy of the host should be higher than that of the dopant. This is generally (but not always) valid for both singlet and triplet excitons. Given the predominance of the triplet excitons generated in the OLEDs, to obtain a device with high luminant efficiency, the triplet energy of organic hosts should be higher than that of the dopant to encourage an exothermic energy transfer from the host to the dopant.
To fabricate emissive layers and corresponding OLEDs using blue and blue-green phosphorescent dopant-emitters such as Ir or Pt complexes requires hosts with high triplet energies (3E=2.7 eV) to achieve an exothermic energy transfer from the host to dopant and prevent back-transfer from the dopant to the host.
An example of a blue-emitting dopant utilizing Ir(III) center is iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate, denoted FIrpic, which has the structure of Formula III:

FIrpic displays a blue phosphorescence spectra corresponding to a triplet energies of 3E=2.65 eV. D'Andrade, B; Thompson, M. E.; Forrest, S. R. PCT Int. Appl. 2002, WO 2002091814; Tokito, S.; Iijima, T.; Suzuki, Y.; Kita, H.; Tsuzuki, T.; Sato, F. Appl. Phys. Lett. 2003, 83, 569.
From the comparison of triplet energies of CBP (3E=2.56 eV) and FIrpic (3E=2.65 eV) it appears that an emissive layer composed of CBP host and FIrpic dopant that the triplet energy transfer is likely to occur from the FIrpic dopant to the CBP host, which is not desirable as the triplet excitons partly migrate away from the emissive dopant.
Currently, the successful application of dopants that display deep-blue phosphorescence is partly precluded by an insufficient availability of hosts that display high triplet levels (3E>2.70 eV).